Authigenic magnesian calcite, dolomite, and aragonite are precipitated in the uppermost terrigenous sediments of the Washington/Oregon accretionary prism by subduction-induced dewatering. These distinctive carbonates are methane-derived and occur at sites of concentrated pore-water expulsion. Unique biologic communities that subsist, at least indirectly, on methane (Suess et al., 1985) are also found at some of these sites. The methane, which is dominantly biogenic, is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement. Carbonate precipitation occurs below the oxic layer, probably no deeper than several centimetres to a few metres below the seabed. Cementation may be induced by three factors: (1) increased carbonate alkalinity resulting from microbial sulfate reduction, (2) decreased σCO2 solubility resulting from a pressure decrease when the pore water escapes the prism, and/or (3) the addition of Ca2+ and Mg2+ ions from sea water near the sediment/water interface.

The convergent margin setting engenders precipitation of authigenic carbonates in several ways. Compressive stresses induce anomalously rapid compaction and dewatering rates, and they may cause overpressuring in migrating pore water, thereby delaying precipitation of carbonates until pressure is released near the sediment-water interface. Structural deformation of the accretionary prism creates pathways (such as fault zones), secondary fracture porosity, and dipping permeable layers (often exposed by mass movement) for efficient advection and expulsion of methane-enriched pore water. These characteristic conditions, which lead to the precipitation of methane-derived carbonates, may be found at other convergent margins.

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